The invention relates generally to a current limiter and more specifically to a superconducting fault current limiter.
Current limiting devices are critical in electric power transmission and distribution systems. For various reasons such as lightening strikes, short circuit conditions can develop in various sections of a power grid causing sharp surge in current. If this surge of current, which is often referred to as fault current, exceeds the protective capabilities of the switchgear equipment deployed throughout the grid system, it could cause catastrophic damage to the grid equipment and customer loads that are connected to the system.
Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current limiting device because of their intrinsic properties that can be manipulated to achieve the effect of “variable impedance” under certain operating conditions. A superconductor, when operated within a certain temperature and magnetic field range (i.e., the “critical temperature” (Tc) and “critical magnetic field” (Hc) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the “critical current level” (Ic)), and is therefore said to be in a “superconducting state.” However, if the current exceeds this critical current level the superconductor will undergo a transition from its superconducting state to a “normal resistive state.” This transition of a superconductor from its superconducting state to a normal resistive state is termed “quenching.” Quenching can occur if any one or any combination of the three factors, namely the operating temperature, magnetic field or current level, exceeds their corresponding critical level. Mechanisms, using any one or any combination of these three factors, to induce and/or force a superconductor to quench is usually referred to as a trigger mechanism.
A superconductor, once quenched, can be brought back to its superconducting state by changing the operating environment to within the boundary of its critical current, critical temperature and critical magnetic field range, provided that no damage was done during the quenching of the superconductor. HTS material can operate near the liquid nitrogen temperature 77 degrees Kelvin (77K) as compared with low-temperature superconducting (LTS) material that operates near liquid helium temperature (4K). Manipulating properties of HTS material is much easier because of its higher and broader operating temperature range.
For some HTS materials, such as BSCCO, YBCO, and MgB2 there often exists, within the volume of the superconductor, non-uniform regions resulting from the manufacturing process. Such non-uniform regions can develop into the so-called “hot spots” during the surge of current that exceeds the critical current level of the superconductor. Essentially, at the initial stage of quenching by the current, some regions of the superconductor volume become resistive before others do due to non-uniformity. A resistive region will generate heat from its associated i2r loss. If the heat generated could not be propagated to its surrounding regions and environment quickly enough, the localized heating can damage the superconductor and could lead to the breakdown (burn-out) of the entire superconductor element.
U.S. Pat. No. 6,664,875, issued on Dec. 16, 2003, entitled, “Matrix-Type Superconducting Fault Current Limiter” (MFCL) assigned to the assignee of the present invention, incorporated by reference in its entirety, uses a mechanism that combines all three of the quenching factors of the superconductor, namely current, magnetic field and temperature, to achieve a more uniformed quenching of the superconductor during current limiting. This MFCL concept can dramatically reduce the burnout risks in bulk superconducting materials due to the non-uniformity which exists in the superconductor volume. In addition, the detection of a fault and subsequent activation of the current-limiting impedance of the MFCL are done passively by the built-in matrix design, without assistance of active control mechanisms. This makes a fault current limiter based on the MFCL concept more easily designed, built and operated for a wide range of potential current-limiting applications.
US Publication US2005/0099253A1, published on May 12, 2005, discloses a superconducting current limiting device comprising a superconductor body electrically connected in parallel with a shunt coil wherein the shunt coil is in tight contact with the external surface of the superconducting body. The shunt coil has an external shape to allow a circular current to flow. This publication does not disclose arranging the shunt coil so that it is not in tight contact with the external surface of the superconductor and yet function as a fault current limiter, nor does this publication disclose arranging the shunt coil at a predetermined range of angles to produce a circulating current.
U.S. Pat. No. 6,043,731, issued on Mar. 28, 2000, discloses a current limiting device having a superconductor, a shunt coil wrapped around the superconductor and connected in parallel with the superconductor, wherein the shunt coil generates a magnetic field to assist in quenching the superconductor. The shunt coil is controlled by active means. This patent does not disclose arranging the shunt coil at a predetermined range of angles to produce a circulating current. Nor does this patent disclose loosely winding the shunt coil around the superconductor.
U.S. Pat. No. 6,809,910, issued on Oct. 26, 2004, discloses a current limiting device having a superconductor, a shunt coil wrapped around the superconductor and connected in parallel with the superconductor, wherein the shunt coil generates a magnetic field to assist in quenching the superconductor. This patent does not disclose arranging the shunt coil at a predetermined range of angles to produce a circulating current, nor does this patent disclose loosely winding the shunt coil around the superconductor.
A magnetic field can be used to trigger High Temperature Superconducting (HTS) materials to improve speed and uniform quenching during transition from superconducting to normal resistive state. Excessive heating in HTS materials, caused by high fault currents, is minimized by using a shunt impedance to divert current from the HTS elements to the shunt impedance. In some Superconducting Fault Current Limiter (SCFCL) designs two external windings (coils) are used, one to generate the trigger magnetic field and one as a shunt impedance. The large number of components (parts) due to the use of two coils per HTS element adds to the complexity of the design and is problematic in areas of manufacturability, size, weight, winding and interconnection power loss, and high voltage design. The use of a single coil for both triggering and shunt reduces the complexity of the design.
As the need for higher power and higher voltage applications of Fault Current Limiters increases, designing a device with less complexity and yet using magnetic field for triggering becomes a challenge. Optimizing fault current limiter design with fewer overall components is important to design a reliable high voltage device at a transmission system level. As the power and voltage requirement increases, the number of components (superconductors and magnetic field coils) increase, which adds to the complexity of the device. Reducing the number of parts is one of the ways to improve reliability of the device. There therefore exists a need for a simplified design with improved reliability in a SCFLC device for transmission system applications.